• Review: Genes on separate chromosomes assort independently of one another during gamete formation. For two genes that code for two different traits, the offspring that result from this independent assortment will display a probable 9:3:3:1 phenotypic ratio. • Two linked genes can produce only two kinds of gametes. Two unlinked genes can produce four kinds of gametes. • Linked genes cannot independently assort from one another, and the resulting offspring do not display a 9:3:3:1 phenotypic ratio. - Our 23 pairs of chromosomes contain thousands of genes, and many genes must share the same chromosome. - Linked genes are located on the same chromosome. - Review: The alleles of genes that are on separate chromosomes (unlinked genes) assort independently of one another during meiosis. - Linked genes do not assort independently. For example, the alleles for eye color and wing shape in a fly are located on the same chromosome. A fly with a ppBB genotype can only form pB gametes, and a PPbb fly can only form Pb gametes. When the two mate, the resulting generation can make only pB or Pb gametes because the unlinked genes cannot assort independently. - Review: If the two genes were located on different chromosomes, a cross between two heterozygous individuals (PpBb) in the F1 would yield an expected phenotypic ratio of 9:3:3:1 in the F2 generation. Each F1 heterozygote will produce four different gametes—PB, Pb, pB, and pb. The gametes would randomly combine, producing four phenotypes in the 9:3:3:1 ratio. - For linked genes, the heterozygotes only produce two kinds of gametes. The composition of the gametes depends on the allelic combinations in the parent (P) generation. The p and B alleles occurred together in one of the parents. The P and b alleles occurred together in the other parent. The F1 individuals produce gametes with the same combinations—pB and Pb. - A Punnett square of the F1 gametes is shown on the left. The expected outcome in the F2 generation is a 1:2:1 ratio of phenotypes. One-fourth are expected to be pink and normal, half will be red normal, and one-fourth will be red and blistery

How do we know that there must be more than one gene on a chromosome?

- There are more genes than there are chromosomes.

Trace the inheritance pattern of two linked genes in fruit flies. Red eye color (P) is dominant to pink (p). Normal wings (B) are dominant to blistery (b). If two heterozygous flies are crossed, what phenotypic ratio represents the most reasonable one expected in the offspring?

1/2 red normal 1/4 pink normal 1/4 red blistery

Which of the following is true regarding Mendel’s experiments with pea plants?

- The inheritance factors he chose to examine were located on separate chromosomes.

If blonde hair and blue eyes are consistently inherited together, the best explanation would be that

- The genes are linked on the same chromosome.

What are linked genes?

- Genes that are located on the same chromosome.

Which one of the following provides evidence that some genes are linked?

- In fruit flies, two parent flies that are heterozygous for two different characteristics produce offspring with a 1:2:1 phenotypic ratio.

9.8.1 Linked Genes Flashcards by Irina Soloshenko

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9.8.1 Linked Genes Flashcards

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